THE son of CBS White House correspondent Bill Plante is cooling his heels in a Martha's Vineyard jail cell for allegedly making a bomb threat, and cops are mystified as to the motive. Cops in Edgartown say John Patrick Plante, 51, a bon vivant who spends the winter in Santa Barbara, Calif., phoned them Oct. 14 to warn that an explosive device had been placed in a bar/restaurant called the Navigator. "We haven't quite figured out why, because it was a bomb threat to an empty building - the Navigator is a seasonal bar open only in the summer," a...